The presently described technology relates generally to methods for making sulfonated polymers. More specifically, the presently described technology relates to the sulfonation of aromatic-containing polymers (e.g., styrenic block copolymers) in non-halogenated aliphatic solvents. In some preferred embodiments, the present technology relates to methods for sulfonating aromatic-containing block copolymers having at least two polymer end blocks that are resistant to sulfonation and at least one polymer interior block that is susceptible to sulfonation.
The presently described technology uses sulfonation reagents such as acyl sulfates to sulfonate aromatic-containing polymers in non-halogenated aliphatic solvents. In accordance with the present technology, the initial concentration of a polymer in a reaction mixture comprising a non-halogenated solvent can be kept below a limiting concentration such that high levels of sulfonation of the aromatic-containing polymer can be achieved in a manner that is free of disabling gelation.
Through the years, there have been many modifications made to aromatic-containing polymers (e.g., styrenic block copolymers) to change and improve their properties. One such modification is to sulfonate the polymers. Once a polymer containing sulfonation-susceptible units is polymerized, and if desired, hydrogenated, it can be sulfonated using a sulfonation reagent. The first information on sulfonation of high polymers such as polystyrene (PS) was published before World War II. Ever since, the utilization of sulfonated polymers in various industrial, domestic, and medical applications has been increasing steadily. Sulfonated ionomers were defined as macromolecular compounds containing sulfonic (˜SO3) groups. These compounds are utilized satisfactorily, because of their interesting chemical and mechanical properties, in a number of industrial applications, e.g., the production of compatible blends of non-miscible polymers, the use in ion exchange materials, the use in membranes for reverse osmosis and ultrafiltration, as plasticizers for macro-defect-free concretes, as conductive composites, etc. A general overview of polymer sulfonation can be found in Kucera, F., Jancar, J., Polymer Engineering and Science (1998), 38(5), 783-792.
For example, one of the first sulfonated block copolymers is generally disclosed in U.S. Pat. No. 3,577,357 to Winkler. The selectively sulfonated block copolymer of the Winkler patent was characterized as having the general configuration A-B-(B-A)1-5, wherein each A is a non-elastomeric sulfonated monovinyl arene polymer block and each B is a substantially saturated elastomeric hydrogenated polymer block made from dienes. The block copolymer in the Winkler patent was sulfonated to an extent sufficient to provide at least 1% by weight of sulfur in the total polymer and up to one sulfonated constituent for each monovinyl arene unit. The Winkler patent teaches that the sulfonation reaction is usually carried out while the copolymer is swollen by or dispersed in an inert medium such as a haloalkane. In the examples of the Winkler patent, a polystyrene-hydrogenated polyisoprene-polystyrene triblock copolymer in cyclohexane was treated with a sulfonating agent comprising sulfur trioxide/triethyl phosphate in 1,2-dichloroethane. Sulfonation of the polymer in cyclohexane in the presence of 1,2-dichloroethane was reported to be accompanied by gelation of the reaction mixture, even at a mere 1% polymer concentration and 2.1% sulfur incorporation (corresponding to about 0.66 milliequivalents per gram (meq/g) sulfonic acid).
U.S. Pat. No. 3,870,841 to Makowski et al., in general, discloses the sulfonation of plastic polymers. It teaches that sulfonic acid groups can be introduced into aromatic-containing polymers by direct reaction with a sulfonating agent, which can be, for example, sulfuric acid and chlorosulfonic acid, in halogenated solvents. Preferred sulfonating agents are acetyl sulfate and sulfur trioxide complexes with dioxane, tetrahydrofuran, and trialkyl phosphates. Allegedly, a sulfonation level of about 0.2 to about 10 mol % can be reached. In one example, a t-butylstyrene/isoprene random copolymer was sulfonated in methylene chloride with a triethylphosphate-SO3 complex. The resulting polymer was reported to contain about 4.4 sodium sulfonate groups per 100 monomer units.
Although there are a number of known chemical reagents and routes that can be used to incorporate sulfonic acid groups into sulfonation-susceptible polymers, the difficulty of sulfonating polymers without gelation is widely appreciated in the art. See Sherrington, D. C.; Swann, A.; Huxham, I. M.; Tetley, L. J. Mater. Chem. 1993, 3, 781, and the references incorporated therein. Gelation of polymers can be caused by chemical gelation, physical gelation, or a combination thereof. Chemical gelation can be caused by polymer cross-linking through formations of covalent bonds such as sulfone cross-linking formations, for example. See Polymer Engineering and Science (1998), 38(5), 783-792. Besides leading to polymer gelation, undesirable chemical cross-linking can also lead to polymer precipitation and/or intractability. Physical gelation, on the other hand, can be caused by non-covalent cross-linking. Physical gelation normally can be disrupted through appropriate solvent conditions. For example, Li, et al. Reactive & Functional Polymers 56:189 (2003) describes the “insolubility” of sulfonated poly[styrene]-block-[2-[(per-fluorononenyl)oxy]ethyl methacrylate] in toluene as being due to “physically cross-linked network in the block copolymer resulting form the intermolecular associations of the ionic dipoles in the system.” It teaches that the addition of polar co-solvent readily enables the dissolution of the polymer.
The literature teaches the use of various acyl sulfates, which can be readily prepared from carboxylic acid anhydrides and sulfuric acid, for the sulfonation of aromatic-containing polymers without the formation of significant sulfone cross-linking groups. Although chemical gelation can be reduced or controlled by the use of acyl sulfates, physical gelation or polymer precipitation still poses a serious problem for polymer sulfonation. To reduce physical gelation or polymer precipitation, the reaction media of choice for the acyl sulfate methods disclosed in the literature are typically halogenated solvents such as dichloroethane. Halogenated solvents are alleged to not only afford solubility to the unsulfonated polymer and the acyl sulfate reagent (e.g., acetyl sulfate), but also to maintain the resulting sulfonated polymer in soluble form (e.g., a homogeneous liquid), without precipitation or disabling gelation. The use of halogenated solvent is, however, highly undesirable from an environmental, health, and safety (EH&S) standpoint. Methods that can effectively sulfonate aromatic-containing block copolymers in non-halogenated aliphatic solvents with equal or greater levels of sulfonic acid incorporation than in halogenated solvents would be highly desirable. Advantages of non-halogenated aliphatic solvents include, for example, (a) not suffering from the substantial environmental concerns associated with halogenated solvents; (b) typically being used in the preparation of the starting block copolymers, thereby enabling the sulfonation of polymer without the need for polymer isolation and re-dissolution prior to sulfonation; and/or (c) being suitable solvents for subsequent downstream processing of the sulfonated polymer into films, membranes, coatings, and the like.
However, in general, the utility of non-halogenated aliphatic solvents for the sulfonation of styrene containing polymers at increased levels of sulfonic acid incorporation appears to be problematic because the resultant sulfonated polymers, having highly polar sulfonic acid groups, are typically incompatible with the non-polar, non-halogenated aliphatic solvent, thereby resulting in disabling gelation and/or precipitation of the polymers. For example, Sheerington disclosed, in general, the failure of acyl sulfates to enable sulfonation of polymers such as poly(styrene)-poly(hydrogenated butadiene)-poly(styrene) triblock copolymers without gelation. See Sherrington, L. J. Mater. Chem., 1993, 3, 781. For another example, in the Winkler patent discussed above, a polystyrene-hydrogenated polyisoprene-polystyrene triblock copolymer in cyclohexane was treated with a sulfonating agent comprising sulfur trioxide/triethyl phosphate dissolved in 1,2-dichloroethane. Sulfonation of the polymer was reported to be accompanied by gelation even in the presence of the halogenated solvent. In addition, the utility of many sulfonation reagents such as acetyl sulfate in combination with non-halogenated aliphatic solvents appears to be poor due to such sulfonation reagents having little or negligible solubility in the non-halogenated aliphatic solvents, resulting in very poor polymer sulfonation conversion.
To enable polymer sulfonation in non-halogenated aliphatic solvents, methods using higher acyl sulfates with adequate solubility have been developed. For example, the homogeneous sulfonation of polystyrene in cyclohexane with higher acyl sulfates such as lauroyl sulfate has been reported. See Thaler, W. A. Macromolecules, 1983, 16, 623. A treat level of 0.6 milliequivalents per gram (meq/g) lauroyl sulfate is defined in the Thaler paper as a level below which reaction solutions are “very tractable.” Id. Above this level, the Thaler paper describes that “it was difficult to discern whether the polymer was completely soluble.” Id. The Thaler paper also tested the use of propionyl sulfate and butyryl sulfate for sulfonating polystyrene in cyclohexane, but the treat level was very low, and much below the 0.6 meq/g level. The Thaler paper indicates that the solubility of polystyrene decreases with increasing sulfonation. Put in another way, the Thaler paper indicates that treatment levels above 60 milliequivalents (meq) of sulfonation reagent per 100 grams of polystyrene (affording corresponding polymer sulfonation levels of approximately greater than 0.35 meq sulfonic acid per gram sulfonated polymer, or about 3.75% degree of styrene sulfonation) resulted in increasing viscosity and difficulty in discernment of the solubility of the polymer product. The Thaler paper also notes that the by-product carboxylic acid appeared to play an important role in helping to maintain polymer solubility, apparently functioning as a co-solvent.
EP 0 071 347 to Thaler also discloses a process for sulfonating polystyrene and other aromatic-containing polymers using C8 or higher acyl sulfates in non-halogenated aliphatic solvents such as cyclohexane. This patent provides a summary of the lack of solubility of lower acyl sulfates such as acetyl sulfate for sulfonation of polymer aromatic groups in non-halogenated aliphatic solvents to desired sulfonation levels. In addition, although EP 0 071 347 describes the sulfonated polymer products as “gel-free,” the means by which gel is measured in the patent makes it clear that the term “gel” in this context refers to gel particles (typically formed by chemical cross-linking) that cannot be dissolved by the addition of appropriate solvents.
For another example of the use of higher acyl sulfates, the Li article discussed above discloses the use of lauroyl sulfate for the sulfonation of poly[styrene]-block-[2-[(per-fluorononenyl)oxy]ethyl methacrylate], with up to 28% styrene sulfonation, in cyclohexane. See Li, et al. Reactive & Functional Polymers, 56:189. The molecular weights of these polymers in the Li article were quite low, and the state of homogeneity or tractability during the course of sulfonation was not described.
Lower acyl sulfates such as C2-C8 sulfates, especially C2 to C4 sulfates have many advantages over the higher acyl sulfates or other sulfonation reagents. These lower acyl sulfates, as with other acyl sulfates, are capable of sulfonating aromatic rings with negligible sulfone formation, thereby proceeding without substantial chemical gelation. In addition, these lower acyl sulfates can be economically prepared from commercially available anhydrides with simple processing equipment. Furthermore, the lower acyl sulfates are more mass efficient than higher acyl sulfates on a molar basis. Still further, the by-product carboxylic acids of the C2 to C4 acyl sulfates are sufficiently volatile to enable at least partial removal of these acids from the sulfonated polymer product by evaporation methods known to people skilled in the art, and are sufficiently water soluble for effective removal without neutralization by washing methods known to people skilled in the art.
Xie, et al. J. Applied Polymer Sci, 96, 1398 (2005), discloses the use of acetone in combination with cyclohexane and acetyl sulfate to sulfonate highly unsaturated styrene-butadiene-styrene triblock copolymers. The Xie article ascribed gelation to the association of sulfonate groups. It noted that acetone might function to reduce this association. However, sulfonic acid incorporations are reported only up to about 0.45 meq/g. The IR spectra in the Xie article appear to show that the sulfonation occurred on both the polybutadiene segments and the polystyrene segments.
Therefore, there is still a need in the art for a method for producing sulfonated aromatic-containing polymers in non-halogenated aliphatic solvents that (1) is substantially free of polymer precipitation; (2) is free of disabling gelation; (3) can efficiently reach a high degree of sulfonation; and/or (4) uses lower acyl sulfates as the sulfonation reagents.